While safety is important, be sure to choose a pleasant/interesting route. This will make people far more likely to return for another walk. Make sure that there are enough drink/refreshment stops along the way, and choose a good place to end the walk - where people can gather for a drink and perhaps have some food.
Ensure you walk the route before the actual day in order to check walking time to various points and preparing/checking route maps and directions.
Consider whether you wish to introduce some type of novelty to the walk, e.g. some people could go in fancy dress if they want, or you can get famous people involved. You may want to give participants badges, T-shirts or something else to help identify them on the walk and give a sense of shared identity.
At an early stage, as soon as you have planned a route, get any permissions or give any notifications required – e.g from police or local authority. You may want to take out public liability insurance cover or check whether existing school insurance will cover the event. To help delegate some of this work, you may want to consider giving some of these roles to volunteers;
co-ordinator: in overall control of the event and its organisation.
base controller: someone to organise the base control team, to check the walkers in/out on the day.
catering officer: someone to organise refreshments.
publicity officer: someone capable of producing a considerable variety and quantity of posters and other publicity material, maps, sponsor forms, check cards, etc.
route controller: someone to organise way marking and co-ordinate the teams operating along the route. Team leaders/marshals should have – first aid kits, rubbish bags, spare suncream particularly for kids and mobile phones so they can make contact with the base controller in event of an emergency.
route designers: several walkers with local knowledge and map-reading ability .
schools liaison officer: someone willing to visit the school and enthuse the pupils.
Depending on the type of walk, people may well need a map and directions. This could tell people such things as the route, points of interest, toilets and refreshment facilities along the route, and what to do at the end, You can either give this to people on the day, or you can send it out in advance in which case of course you would also tell people such things as where the start is and when to arrive.